Double diffused (DMOS) transistors are used widely in both RF analog and mixed-signal integrated circuits (ICs), and in dielectrically isolated ICs for power and high voltage applications. These devices are produced in both vertical and lateral configurations. In the lateral configuration (LDMOS) the power handling property of the device is enhanced by extending the active portion of the device along the x-y plane of the substrate. In a typical implementation the active region meanders in a serpentine pattern to increase the length to width ratio of the overall device.
The surface electrode configurations for these devices are substantially elongated, and may be relatively complex. In a common power device structure, the elongated gate and drain elements are formed in a comb configuration with the combs facing and the teeth, referred to here and below as fingers, interleaved. The surface electrodes may be drain and gate, or source and gate. In each case the third electrode is provided by the substrate. In state of the art RF LDMOS power devices the surface electrodes are typically drain and gate. In this description, conductors that provide interconnections above the surface of the semiconductor substrate may be referred to as runners.
The length-to-width ratio of the surface runners is related not only to device performance, but also to the ever-present goal of overall device miniaturization. For economy of area, the width of the runners is made as small as practical. This results in a very thin electrode structure wherein the fingers are fragile and susceptible to mechanical defects. Device failure is often attributed to these defects. Mechanical defects may occur in production, or may arise later as a result of stress aging.
Improvement in the mechanical performance of elongated runners would represent a significant advance in LDMOS and related device technology.